The invention relates to the preparation of catalysts components, and to a process, for polymerizing an .alpha.-alkene in the presence of Ziegler-type catalysts that have been obtained by having an organomagnesium compound and an organoaluminumhalide react in the presence of a carrier, by combining the reaction product with a tetravalent titanium compound and/or compound of another transition metal from the groups IV up to and including VI of the Periodic System, in particular a vanadium compound, and by activating the reaction product with an organoaluminum compound after reduction of at least a major part of the transition metal compound.
The present invention relates in particular to the polymerization of ethylene, or copolymerization of ethylene with minor quantities of propylene and/or butylene.
So-called Ziegler-type coordination catalysts for polymerization of .alpha.-alkenes are generally known. Many and various kinds of proposals to improve the properties of such catalysts have become known. However, catalyst systems of this kind are at present still being examined and developed on an extensive scale because of the advantages and disadvantages in relation to improving such properties as the more or less easy preparation, activity of the catalyst and allied polymer yield, reproducibility thereof, and the like.
As a rule, it appears that a given catalyst component, which is suitable in case of a particular catalyst composition, and which then brings about an improvement in one or several respects, is, even in the most favorable case, not only not active for its purposes in use with a different composition, but may also often lead to adverse effects on the catalyst properties or on the product properties of the polymer prepared therewith.
Therefore, no guidelines have or can yet be established for the development of Ziegler-type catalyst compositions for polymerization of .alpha.-alkenes.
This applies not only in relation to the catalyst components and the ratios in which they are present in the catalyst composition, but also in relation to the particular method used for preparing the catalyst. For instance, the sequence used for combining the components, the temperature employed, and like factors may have important influences on the catalyst behavior, but in an unpredictable way.
A great many such catalyst systems are based on the reduction of a transition metal compound, particularly a titanium compound, with an organoaluminum compound, whereupon, generally, the reaction product is thereafter activated with an organoaluminum compound. It has also been proposed to reduce the transition metal compound with an organomagnesium compound, for instance as in U.S. Pat. No. 3,392,159. From this specification it is also known that the sequence of adding the catalyst components may exert a particularly great influence on the ultimate properties of the catalyst system, especially activity which is especially very strongly influenced by order of addition. For instance, through the addition of an organomagnesium compound to a titanium compound, catalysts can be obtained whose activities surpass those of catalyst systems prepared by adding the titanium compounds to the organomagnesium compounds, by several orders of magnitude.
Applicant's British patent specification No. 1,436,426 describes the polymerization of .alpha.-alkenes in a diluent, or liquid monomer, in which the polymerization is carried out in the presence of a catalyst prepared by having an organoaluminum halide compound react with an organomagnesium compound in an inert diluent, and in the presence of a carrier, and by subsequently combining the reaction mixture with a tetravalent titanium compound. If necessary, there may be a washing out and/or heating of the catalyst suspension formed after the reduction of at least a major part of the tetravalent titanium, and then activating of the resulting reaction product with an organoaluminum compound. In the preparation of that catalyst the alkylaluminumhalide and the magnesium compound are both introduced into the diluent in which the carrier has already been suspended. Next, a solution of TiCl.sub.4 is slowly added thereto. The reaction product which thus forms is thereafter activated with an organoaluminum compound.
Though catalysts prepared in this way are highly active, the characteristics of the polyalkene powder prepared therewith, in particular of polyethylene, still may be improved, especially because of rather broad particle size distributions and too high a content of fines, i.e., particles having a particle size of less than 70.mu..
Variations in the particle size of polyalkenes cause problems in their use, including difficulties in handling the same in automatic feeding equipment, and non-uniform performance during film-forming, casting, molding or other operation.